Norepinephrine and dopamine-beta-hydroxylase (D beta H), which are released when sympathetic nerves are stimulated, were studied as indicators of sympathetic nerve activity. Mean plasma norepinephrine levels were higher in 28 sustained or intermittent hypertensives than in matched normotensives (hypertensives 0.25, normotensives 0.18 ng/m1, p less than 0.02); mean plasma levels of D beta H activity were 42 and 35 units respectively (p greater than .27) in the same subjects. Plasma D beta H activity showed a wide scatter in both groups and correlated poorly with either plasma norepinephrine levels or diastolic blood pressure. D beta H activity in plasma of seven patients with secondary hypertension was indistinguishable from levels in the other groups. Plasma levels of norepinephrine doubled after the subjects had been erect for ten minutes whereas plasma D beta H activity, corrected for small changes in the concentration of plasma proteins, remained unchanged. It is concluded that plasma D beta H activity is an insensitive index of sympathetic nerve function; the findings suggest increased sympathetic nerve activity in the subjects with essential hypertension.